1. Field of the Invention
The present invention generally relates to an apparatus and method for transmitting/receiving a signal in a communication system using a low density parity check (LDPC) code, and more particularly to an apparatus and method for transmitting/receiving a signal according to partial incremental redundancy (IR) scheme in a communication system using an LDPC code.
2. Description of the Related Art
The most fundamental problem in communication is on how efficiently and reliably data can be transmitted over a channel. In the next generation multimedia mobile communication systems, on which extensive research is being conducted, it is mandatory to increase the system efficiency using a proper channel-coding scheme, as the systems are required to process and transmit a large amount of data such as images, wireless data, etc. beyond a basic voice service.
However, when data is transmitted, information loss may result from inevitable error due to noise, interference and fading according to channel condition. To reduce this information loss, various error control schemes are used according to channel characteristics, such that the system reliability is improved. Among the error control schemes, the most basic scheme is an error correction code scheme. The error correction code scheme will be referred to as the forward error correction (FEC) scheme. Typical error correction codes are a turbo code, a low density parity check (LDPC) code, etc.
The error control schemes include an automatic retransmission request (ARQ) scheme as well as the FEC scheme. The ARQ scheme can obtain the relatively high system reliability with a relatively simple structure. The above-described FEC scheme corrects error of received information using a code with error correction capability. The FEC scheme is used when no feedback channel is present to notify a transmitter if a receiver has successfully received information.
Because no feedback channel is present when the FEC scheme is used, uncorrected data may be transferred if an error correction process for information received by the receiver fails, resulting in performance degradation. The ARQ scheme is used to provide the transmitter with a retransmission request for information in which an error has occurred when the receiver detects an error in the received information using a cyclic redundancy check (CRC) code with superior error detection capability.
There has been proposed a hybrid automatic repeat request (HARQ) scheme corresponding to a new error control scheme in which advantages of the ARQ and FEC schemes are combined. The HARQ scheme reduces the number of retransmissions by correcting a relatively frequent error pattern using the FEC scheme. The HARQ scheme is divided into two types, i.e., type I and type II.
FIG. 1 illustrates a signal transmission operation according to a type-I HARQ scheme in a conventional communication system.
The type-I HARQ scheme is referred to as a chase combining (CC) scheme. In the type-I HARQ scheme, a transmitter transmits signals of the same format upon initial transmission and retransmission and a receiver receives, soft combines the initially transmitted and retransmitted signals, and decodes the soft combined signals. In the type-I HARQ scheme, the transmitter transmits a complete codeword in both the initial transmission and the retransmission. The codeword is a channel codeword, for example, an error correction codeword, and is constructed with an information part corresponding to an information bit stream and a parity part corresponding to parity, i.e., a parity bit stream.
The initial transmission is a first transmission and the retransmission is a subsequent transmission including a second transmission.
Referring to FIG. 1, the transmitter transmits signals of the same format upon initial transmission and retransmission when the type-I HARQ scheme is used. When the receiver has not normally received a signal initially transmitted from the transmitter, i.e., the transmitter detects a retransmission request for the initially transmitted signal, the transmitter retransmits a signal of the same format as that of the initially transmitted signal.
When receiving the signal initially transmitted from the transmitter, the receiver decodes the received signal and checks for errors using the cyclic redundancy check (CRC) code check, etc. When an error has occurred in the received signal, the received signal is buffered. Then, the receiver sends an associated signal retransmission request by sending non-acknowledgement (NAK) information indicating that an error has occurred in the signal initially transmitted by the transmitter. The receiver receives a signal retransmitted from the transmitter in response to the retransmission request, soft combines the buffered signal and the retransmitted signal, and decodes the soft combined signals.
When the same signal, i.e., the total codeword, is transmitted in both the initial transmission and retransmission according to type-I HARQ scheme, there is provided only reception gain rather than coding gain according to channel state.
FIG. 2 illustrates a signal transmission operation according to type-II HARQ scheme in the conventional communication system.
The type-II HARQ scheme is referred to as an incremental redundancy (IR) scheme. In the type-II HARQ scheme, a transmitter transmits signals of different formats upon initial transmission and retransmission and a receiver code combines and decodes the initially transmitted and retransmitted signals. Among currently proposed HARQ schemes, the type-II HARQ scheme provides maximal throughput. The initial transmission is a first transmission and the retransmission is a subsequent transmission including a second transmission.
Referring to FIG. 2, the transmitter transmits signals of different formats upon initial transmission and retransmission when the type-II HARQ scheme is used. When the receiver has not normally received a signal initially transmitted from the transmitter, i.e., the transmitter detects a retransmission request for the initially transmitted signal, the transmitter provides the receiver with a signal different from the initially transmitted signal. In this case, the signal initially transmitted from the transmitter is an information part of a pre-generated codeword and a partial parity bit stream within a parity part. The signal retransmitted from the transmitter is a partial parity bit stream of the remaining parity stream excluding the initially transmitted parity bit stream.
When receiving the initially transmitted signal from the transmitter, the receiver decodes the received signal and checks for errors. When an error has occurred in the received signal, the received signal is buffered. Then, the receiver sends an associated signal retransmission request by sending NAK information indicating that an error has occurred in the signal initially transmitted by the transmitter. The receiver receives a signal retransmitted from the transmitter in response to the retransmission request, soft combines the buffered signal and the retransmitted signal, and decodes the soft combined signals.
When the type-II HARQ scheme is used, the receiver obtains coding gain by soft combining the signals initially transmitted and retransmitted from the transmitter and decoding the soft combined signals. When receiving the signal retransmitted by the transmitter in the type-II HARQ scheme, the receiver code combines and decodes the initially transmitted signal, i.e., an information bit stream and a partial parity bit stream, and a new additional parity bit stream, thereby obtaining coding gain because the signals are decoded at a coding rate that is less than that upon initial transmission.
When the type-II HARQ scheme is used, a codeword is a factor significantly affecting its performance. A design of a rate-compatible code for supporting a superior variable coding rate acts as an important factor for increasing the efficiency of the type-II HARQ scheme.
A general LDPC code is defined by the degree of a check node and the degree of a variable node. As a coding rate decreases, the check node degree decreases and the variable node degree increases. An LDPC code structure is limited to performing coding and decoding processes at a high rate in an actual communication system. In the actual communication system, a maximal variable node degree is limited for fast decoding and a minimal check node degree is limited by the structure of a structured LDPC code, for example, a repeat accumulate (RA) type-LDPC (RT-LDPC) code, for fast coding.
The RT-LDPC code can be an RA code, a concatenated zigzag (CZZ) code, an irregular repeat accumulate (IRA) code, an irregular CZZ (ICZZ) code, a block-type LDPC (B-LDPC) code, etc.
FIG. 3 illustrates the Tanner graph of the conventional RT-LDPC code.
As illustrated in FIG. 3, the Tanner graph of an LDPC code structured for fast coding, i.e., an RT-LDPC code, shows a form in which one check node necessarily connects to two parity nodes. Thus, a coding rate capable of being supported in the RT-LDPC code is also limited. For example, assuming that a maximal variable node degree is limited to a value of less than 7, a minimal coding rate capable of being supported in the RT-LDPC code becomes 1/7. This is a coding rate of a regular LDPC code with a degree of 6 when all information nodes are variable nodes. However, when an irregular LDPC code is generated to improve the performance of the RT-LDPC code, the coding rate exceeds 1/7. Thus, when the variable node degree is limited to 6, a minimal coding rate capable of being supported in the irregular LDPC code becomes about ⅕˜⅙.
When the variable node degree is limited to [PLEASE CONFIRM.] less than 7, a minimal coding rate capable of being supported in the irregular LDPC code becomes about ⅕˜⅙. Thus, the minimal coding rate capable of being supported becomes about ⅕˜⅙ even when the type-II HARQ scheme is used. However, when a channel state is bad due to fading in the actual communication system, a coding rate of less than about ⅕˜⅙ may be needed.
When the RT-LDPC code for supporting a coding rate of less than about ⅕˜⅙ is needed, a partial CC scheme should be used. The partial CC scheme is a scheme for retransmitting only part of an initially transmitted signal rather than a scheme for retransmitting a signal of the same format as that of an initially transmitted signal as in the CC scheme.
It is preferred that a partial IR scheme corresponding to a combination of the IR scheme and the partial CC scheme is used to support coding rates ranging from a high coding rate to a low coding rate using one RT-LDPC code.
FIG. 4 illustrates a signal transmission operation according to partial IR scheme in the conventional communication system.
Referring to FIG. 4, the partial IR scheme uses the IR scheme at coding rates ranging from a maximal coding rate to a minimal coding rate capable of being supported in the communication system and uses the partial CC scheme when a coding rate less than the minimal coding rate needs to be supported.
As described above, because the partial IR scheme also uses the IR scheme, a design of a codeword for supporting a superior variable coding rate acts as an important factor for improving the performance. In particular, a design of a codeword, i.e., an RT-LDPC codeword, for supporting the variable coding rate is very important in the communication system using an RT-LDPC code. Thus, a need exists for a method for efficiently transmitting/receiving an RT-LDPC codeword for supporting a variable coding rate to improve the performance in a communication system using a partial IR scheme.